I appear to have an increasing problem when replaying recorded items in that the unit 'freezes' for up to a minute before continuing without apparent loss of any signal.
However a screen message comes up saying
'Channel scrambled or not available'
If I rewind the section, it will replay without a break but a bit later further freezes occur.
Whichever channel is recorded makes no difference.
The unit is played through a Samsung UE37ES6300UXXU.
The unit was bought in Nov 2011

Any thoughts gratefully received.

I would guess the problem is marginal sector on the hard drive. You could either format the drive (which will delete all recordings) or install the custom firmware and use maintenance mode to run fix-disk.

I think this unit has a two year warranty, if you are having problems contact your retailer. Do not tinker with it as you may invalidate that warranty.

Thanks for your comments which confirm my own thought of an HD problem.
The unit was bought from Humax Direct so I'll be giving them a ring since it has a 2 year warranty.